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Canine blastomycosis: A review and update on diagnosis and treatment

This fungal disease, which readily infects dogs and people, typically starts out in the lungs but can go on to invade many tissues throughout the body. Identifying it quickly and implementing antifungal therapy can result in a good prognosis.

Blastomycosis is a systemic fungal infection caused by the dimorphic fungus Blastomyces dermatitidis. The infective form of the organism, the mycelial phase, is most likely to be found in sandy, acidic soil near bodies of fresh water.1-5 High organic matter content in the soil from decaying wood byproducts or animal waste together with moist conditions promotes growth of the organism.1Blastomyces dermatitidis has a relatively wide distribution in North America, including the Mississippi, Missouri, and Ohio river valleys; the Middle Atlantic states; southern Saskatchewan; Manitoba; Quebec; and Ontario.1,4,6 Blastomycosis is most commonly diagnosed in dogs and people.1

RISK FACTORS

Dogs at greatest risk for developing clinically apparent blastomycosis are 2- to 4-year-old intact male large-breed dogs living in endemic regions.1,2,3,5 This group of dogs has a greater tendency to roam and to sniff and dig in the soil, resulting in greater exposure to the organism. Sporting dogs and hound breeds are predisposed, most likely because of increased exposure to high-risk areas during hunting.4,5 Residence near a river or lake and access to recently excavated sites have been demonstrated to increase the risk of infection.7,8 Most cases of canine blastomycosis are diagnosed in late summer or early fall.2,5

ROUTE OF INFECTION

Infection most commonly occurs after inhaling spores from contaminated soil.1-3 At normal canine body temperature, the organism transforms to a yeast that can infect the lungs and spread systemically. Although infection almost always begins in the lungs before being disseminated through hematogenous or lymphatic routes to other body tissues, lung lesions occasionally resolve by the time infection in other sites becomes apparent.1-3 The most common sites of clinically apparent infection in dogs include the lungs, lymph nodes, eyes, skin, and bone.1,2,4,9 Subclinical or spontaneously resolving infection is uncommon.1

DIAGNOSIS

Clinical findings in dogs with blastomycosis reflect the systemic inflammatory response and the site or sites of infection. Complete blood count and serum chemistry abnormalities are usually nonspecific and reflect chronic inflammation. Thoracic imaging should be done in all dogs with suspected blastomycosis. Cytologic or histologic examination of infected tissues may reveal the organisms. Serologic, urinary antigen, and polymerase chain reaction (PCR) tests are also available.

Clinical signs

Nonspecific signs of illness, including anorexia, weight loss, and lethargy, are common, and fever (temperature > 103 F [39.4 C]) is present in 40% to 60% of infected dogs.1

Lungs. Lung pathology occurs in 65% to 85% of cases, often resulting in exercise intolerance, cough, tachypnea, cyanosis, or respiratory distress.1-4,9 Lung lesions may also be clinically silent, so a thoracic radiographic examination is recommended in all dogs suspected of having blastomycosis.1

Lymph nodes. Enlargement of one or more peripheral lymph nodes occurs in 30% to 50% of infected dogs, reflecting either reactive hyperplasia or infection of the lymph node by Blastomyces organisms with resultant pyogranulomatous inflammation.1,2,10,11

Figure 1. Panophthalmitis in a dog with ocular blastomycosis.

Eyes. Ocular lesions are identified in 20% to 50% of cases, with endophthalmitis being the most common abnormality.1,12,13 Rapid diagnosis and treatment are essential to preserve vision, so aggressively investigate whether blastomycosis is present in all dogs from endemic regions with early signs of uveitis, including conjunctivitis, iridial hyperemia, aqueous flare, and miosis. Glaucoma secondary to iridocorneal angle obstruction may occur. Other ocular manifestations of blastomycosis may include corneal edema, chorioretinitis, optic neuritis, serous or granulomatous retinal detachment, hyalitis, and vitreal hemorrhage.1-4,12 Panophthalmitis with associated orbital and periorbital inflammation may also develop (Figure 1).1-4,12